Typically, manufacture of wafers and solar cells, include processes, such as slicing, cleaning, damage etching, texturization, diffusion, oxide etching, anti-reflection coating, metallization and so on. The slicing process is generally the first step in any solar cell manufacturing line. Wire saws are extensively used to slice crystalline silicon blocks to produce wafers.
Besides slicing silicon, the wire saws are also used for slicing a variety of other materials including sapphire, gallium arsenide (GaAs), indium phosphide (InP), silicon carbide (SiC), glass, lithium tantalate (LiTaO3) Z-cut crystals, lithium niobate (LiNbO3), lithium triborate (LiB3O5), quartz crystals, ceramics like aluminum nitride (ALN) and lead zirconate titanate (PZT), magnetic materials/parts, optical parts and the like material.
The wire saws typically use a 120-180 micron diameter steel wire, which is several hundred kilometers long (FIG. 1). The wire is wound around a supply spool 110, a set of rollers called “wire guides” 130 to make a bed of parallel moving wire, often called “wire web” 140, and a take-up spool 120 as shown in FIG. 1. The wire guides 130 have equally spaced grooves on their outer surface to control spacing between the wires as it goes around the wire guides 130. The distance between the grooves, called pitch, eventually decides thickness of the wafers.
During the manufacture of wafers, the work piece or the ingot 150, which needs to be sliced, is first glued to a plate 160 and then mounted on the wire saw. Then the ingot 150 is pressed with a vertical motion (top to bottom or bottom to top) against the horizontally moving wire web 140 in a wire saw to slice wafers which still adhere to the plate 160 after the sawing process. During the sawing process, the wire travels at a speed of about 10-15 meters/sec (or even higher) during slicing of wafers. Abrasive slurry, mainly made up of silicon carbide grains and a lubricant (e.g., polyethylene glycol or mineral oil), is introduced over the wire web 140. The abrasive slurry 210 coats the wire and travels to the cutting zone as shown in FIG. 2. Typically, slicing is achieved by slowly pushing the ingot 150 against the wire web 140. Furthermore, as cutting progresses, very fine silicon particles are loaded into the slurry. These particles in the slurry can increasingly adhere to the wafer surface as a function of time during the process. This is particularly true for very thin wafers, which require a much longer time to cut.
In addition, the current wire saws generate heat during slicing. Also, as the wafers become thinner, the cutting surface area increases significantly and as a result this can significantly increase the amount of heat generated during slicing. Further, the current wire saws cannot dissipate such heat generated during slicing. Furthermore, lesser area is generally available for heat dissipation by radiation during slicing due to the slurry getting loaded between the wafers. This can lead to significant thermal stress in the wafers. Furthermore, the heat generated during slicing can soften the glue holding the stack of wafers to the plate 160. This can result in wafers dislodging from the plate 160 and breaking during slicing.
Although the aforementioned conventional wire saws are widely used for slicing brittle materials such as silicon, they also impose a series of high stress operating conditions during slicing. Typically, the mechanical stresses are due to pressure and vibration on the wire as well as hydro-dynamic stresses originating from the slurry. The wafers must withstand these forces during operation otherwise they will break. This may pose serious challenge to the solar industry to slice thinner wafers. As the silicon wafers are manufactured to thinner specification, the sensitivity of each wafer to any stress is increased and wafers readily break.
Currently, the standard for the solar industry is wafers sliced to a thickness of about 200 micrometers (microns; μm). An industry road map calls for reducing this thickness to 100 micron in the next 2-3 years. New processes to reduce mechanical and hydro-dynamic stresses during wafering and subsequent handling steps must be found to achieve this target. In summary, the mechanical, thermal and hydrodynamic stresses induced during the above slicing process can result in significant breakage of silicon wafers and thereby increasing the cost of manufacturing silicon wafers. Further, as the silicon wafers are manufactured to thinner specifications, the sensitivity of these thinner wafers to any stress is significantly increased and these wafers can readily break.
In the wire saw, slicing is completed when the ingot 150 completely passes through the wire web 140 in the wire saw. At this point, the wafer stack which is held to the plate 160 is slowly pulled out of the wire web 140. After completing slicing and removing the stack of wafers from the wire saw the wafers are then cleaned immediately with water and other solvents to remove the abrasive slurry 210, otherwise the abrasive slurry 210 may stain the wafers thereby making them unusable in downstream processes. Further, the slurry remaining between the wafers needs to be removed quickly otherwise the slurry between the wafer can harden and hold the wafers together tightly and can make it difficult to remove the wafers from the plate 160 and in some instances can break the wafers.
Furthermore, removing the wafers from the plate 160 is generally a very labor intensive task as the glue which holds the wafers to the plate 160 is softened by heating and also that the wafers are removed individually (mostly manually). Furthermore, the process requires rearranging of the sliced wafers after the sawing by manually detaching each wafer from the plate 160 and loading them into plastic wafer carriers or cassettes before cleaning. This can be a tremendous amount of manual task. Moreover, this labor intensive task can lead to certain amount of breakage of sliced wafers during manual handling. In a solar manufacturing line, these plastic wafer carriers including the wafers are passed through a series of cleaning and etching tanks containing water and chemicals to remove dirt and damaged layers from the wafer surfaces.
Finally, these plastic wafer carriers including the wafers are immersed in an appropriate chemical to texturize the wafer surface to increase sunlight absorption. The texturized wafers are then dried and transferred from plastic wafer carriers to ceramic or quartz wafer carriers for high temperature treatment (up to 1500° C.) in diffusion furnaces. It can be seen that the above solar manufacturing process is very labor intensive and slow. Also, it can be seen that the above described solar manufacturing process may require a significant amount of equipments.
In addition, in above described current conventional downstream processes that follows the slicing and cleaning steps, the wafers are handled individually, where the wafers are picked, turned, rinsed, dried, flipped, carried and stacked several times before they are transformed into solar cells. All these steps can be very time consuming and can carry tremendous risk of wafer breakage. Generally, in the cleaning and etching processes, wafers are placed in plastic carriers/holders and dipped in a solution. These wafer carriers are typically designed to hold about 25-200 wafers. The number of wafers that can be held in each carrier/holder is typically limited by the gap needed to be maintained between wafers in the carrier/holder. It can be seen that a typical solar cell manufacturing line can require a significantly large number of wafer carriers to keep the process running without any interruption.
Typically, in the manufacture of wafers and cells for photovoltaic applications, thousands of wafers are processed per hour, which necessitates installing a large number of chemical stations to achieve a desired throughput. Thus, increasing the number of wafers per carrier/holder can significantly increase wafer throughput. Further, during the chemical etching process, thin wafers tend to float to the top surface of the etching solution, thereby resulting in not completely treating the wafers.
Furthermore, during diffusion and coating processes, the gases are blown over the thin wafers. The force generated by the blown gases can dislodge the wafers from the wafer carrier affecting productivity. Moreover, the above problems can become very acute when wafer thicknesses are reduced to 100 um (micrometer) or thinner. It can be seen that as the wafers and the subsequent solar cells become thinner, using the above described conventional process can significantly increase the breakage rate of wafers. This can result in increased cost for producing solar cells.